"string" is a type that only makes sense inside the context of a specific programming language, because programming languages differ on what a string is and how it behaves (is it guaranteed to be valid UTF8, does it carry its own length around with it, is it runes/graphemes and can't be split on byte boundaries easily, is it mutable, etc).

Double quotes are a syntax sugar for string literals in source code, in a particular language, to avoid writing `string.from_byte_array([97, 98, 99])` or `new String({97, 98, 99});` or whatever. Strings are single quoted not double quoted in Dyalog APL, there's several different strings in SWI Prolog depending on using double quotes or backticks and how the flags are set.

stdout is an untyped byte stream which could go to a printer or Braille terminal or anything as you say, but could be terminal control codes, image data, or whatever. The OS doesn't tend to have a 'string type' ... but in PowerShell `write-output "a"` will write a .NET System.String to the output stream, but it won't use "printf()".